The present invention relates to a data transmission system over a building or house and, more particularly, to such a data transmission system using a polling system.
It is known in the art that a data transmission system can be suitable for use as a gas central monitoring system etc. over a building or house. Any central monitoring system comprises a plurality of sensors and actuators, which are spread over the building or house. Data developed from the sensors and the actuators are gathered and provided to a central machine to develop any necessary alarm. For a reduction of cost, the data should be superimposed on a power line.
However, a fluorescent lamp with a condenser for improving power factor or a high power electric heater may be connected to the power line. Such a lamp or electric heater can show a low impedance to a carrier frequency. Therefore, a signal level can be lowered to thereby prevent correct data transmission.